Mold for producing silicon ingot and method for fabricating the same

ABSTRACT

A mold for producing a silicon ingot having a layered structure comprising an inner silica layer containing at least one layer in which a fused silica powder with a particle size of 100 μm or less and a fine fused silica sand with a particle size of 100-300 μm is bonded with a silica binder, and an outer silica layer containing at least one layer in which a fused silica powder with a particle size of 100 μm or less and a coarse fused silica sand with a particle size of 500-1500 μm is bonded with a silica binder.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a mold for producing a siliconingot for use in producing a polycrystalline silicon ingot to be usedfor a silicon substrate for a photovoltaic solar cell, and to a methodfor fabricating the same. The present invention especially relates to amold for producing a silicon ingot having an inner silica layer, or aninner silica layer and outer silica layer, being free from crack due toinner stress caused by cooling after solidification of the moltensilicon, and a method for fabricating the same.

[0003] 2. Description of the Related Art

[0004] A quartz mold or a graphite mold has been used as a mold forproducing a silicon ingot.

[0005] The quartz mold is fabricated by the steps comprising: preparingfused silica powder, filling the fused silica powder into a cavity of agraphite mold comprising an inner mold and outer mold having a cavitywith the same shape as the mold; rotating the outer mold while firing aburner flame on the fused silica powder adhered on inner side wall ofthe outer mold after removing the inner mold; and forming the silicapowder into a mold shape while the powder is melting. The cross sectionof the conventional quartz mold fabricated as described above is shownin FIG. 9. otherwise, the graphite mold is fabricated by assemblinggraphite plates. Because detachability of the ingot from the mold is sopoor, sometimes causing cracks in the ingot, when the mold is directlyused after fabrication that the surface of the mold should be treatedwith an inert coating substance. Accordingly, the inside wall of themold is coated with a slurry prepared by mixing a powder of silicondioxide (SiO₂), silicon carbide (SiC), silicon nitride (Si₃N₄), oryttrium oxide (Y₂O₃) with 4% aqueous polyvinyl alcohol solution as arelease agent at a thickness of 0.5 mm or less.

[0006] However, the silicon ingot 2 is subjected to heat-shrink when themolten silicon is cooled in the quartz mold 1. Since the silicon ingotis heat-shrunk along the direction indicated by S in FIG. 9 whileremaining to be adhered to the wall face of the quartz mold 1, the outercircumference of the silicon ingot 2 suffers a tension at the inner wallof the quartz mold to generate dislocations or cracks due to innerstress inside of the ingot 2. The silicon substrate for use in aphotovoltaic solar cell, produced from the silicon ingot having cracksor dislocations, inevitably has poor photovoltaic efficiency.

[0007] On the other hand, particles of the release agent freed from thesurface of the mold penetrate into the molten silicon when the moltensilicon is allowed to stand for a long period of time in the mold coatedwith the release agent. Consequently, a silicon substrate with excellentphotovoltaic conversion efficiency can not be obtained because particlesof the release agent are enveloped into the silicon ingot produced usingthe mold coated with the release agent.

[0008] Meanwhile, producing a high purity silicon ingot with a morereasonable production cost is crucial cost down requirements for siliconsubstrate production.

SUMMARY OF THE INVENTION

[0009] Accordingly, the object of the present invention is to provide asilicon ingot with fewer cracks, dislocations and no release agent withreasonable cost than the ingot produced by using the conventional quartzmold.

[0010] In one aspect, the present invention provides a mold forproducing a silicon ingot having a layered structure comprising an innersilica layer containing at least one layer in which a fused silicapowder with a particle size of 100 μm or less and fine fused silica sandwith a particle size of 100-300 μm is joined with a silica binder, andan outer silica layer containing at least one layer in which a fusedsilica powder with a particle size of 100 μm or less and coarse fusedsilica sand with a particle size of 500-1500 μm is bonded with a silicabinder.

[0011] In accordance with another aspect, the present invention providesa method for fabricating a mold for producing a silicon ingot having aninner silica layer and an outer silica layer comprising the steps of:

[0012] forming a slurry layer on the surface of a wax mold by dipping,followed by lifting up, the wax mold having the same shape as that ofthe inner shape of the mold for producing a silicon ingot into a slurrycomprising fused silica powder and colloidal silica, followed by formingan inner stucco layer by sprinkling fine fused silica sand with aparticle size of 100-300 μm on the surface of the slurry layer;

[0013] forming an outer stucco layer on the inner stucco layer bysprinkling coarse fused silica sand with a particle size of 500-1500 μmon the surface of the slurry layer after forming the slurry layer byfurther dipping, followed by lifting up, the wax mold on which the innerstucco layer has been formed into a slurry comprising the fused silicapowder and colloidal silica; and

[0014] heat-melting and eliminating the wax mold along with baking theinner stucco layer and outer stucco layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 denotes an illustrative cross section of the mold forproducing a silicon ingot according to the present invention.

[0016]FIG. 2 denotes an illustrative cross section showing thesolidified silicon ingot by cooling after injecting a molten siliconinto the mold for producing a silicon ingot according to the presentinvention.

[0017]FIG. 3 denotes an illustrative cross section of the mold forproducing a silicon ingot according to the present invention.

[0018]FIG. 4 denotes a horizontal cross section of the mold forproducing a silicon ingot according to the present invention having aninner space in the mold at an arbitrary height.

[0019]FIG. 5 denotes an illustrative cross section of the mold forproducing a silicon ingot according to the present invention.

[0020]FIG. 6 denotes an illustrative cross section showing thesolidified silicon ingot by cooling after injecting a molten siliconinto the mold for producing a silicon ingot according to the presentinvention.

[0021]FIG. 7 denotes an illustrative cross section of the mold forproducing a silicon ingot according to the present invention.

[0022]FIG. 8 denotes an illustrative cross section showing thesolidified silicon ingot by cooling after injecting a molten siliconinto the mold for producing a silicon ingot according to the presentinvention.

[0023]FIG. 9 denotes an illustrative cross section showing thesolidified silicon ingot by cooling after injecting a molten siliconinto the conventional mold for producing a silicon ingot.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] The mold 5 for producing a silicon ingot as shown in the crosssection in FIG. 1 has a layered structure comprising an inner silicalayer 3 containing at least one layer in which a fused silica powder(not shown in the drawing) with a particle size of 100 μm or less and afine fused silica sand 31 with a particle size of 100-300 μm are joinedwith a binder, and an outer silica layer 4 containing at least one layerin which a fused silica powder with a particle size of 100 μm or lessand a coarse fused silica sand 41 with a particle size of 500-1500 μm isbonded with a silica binder. The inner silica layer 3 containing thefused silica sand 31 having fine particle size is readily released fromthe outer silica layer 4 containing silica sand 41 having coarseparticle size. The inner silica layer 3 is released from the outersilica layer 4 by being adhered to the silicon ingot 2 when theperiphery of the silicon ingot 2 is pulled toward the inner wall face ofthe mold as shown in FIG. 2 by shrinkage of the mold while the moltensilicon poured into the cavity of the mold 5 for producing the siliconingot is cooled after solidification. Accordingly, neither inner stressnor cracks and dislocations as seen in the ingot obtained using theconventional mold occur in the solidified silicon ingot.

[0025] The mold for producing a silicon ingot—having a layered structurecomprising an inner silica layer 3, containing at least one layer inwhich the fused silica powder (not shown in the drawing) with a particlesize of 100 μm or less and the fine fused silica sand 31 with a particlesize of 100-300 μm are joined with a binder, and an outer silica layer 4containing at least one layer in which the fused silica powder with aparticle size of 100 μm or less and the coarse fused silica sand 41 witha particle size of 500-1500 μm is bonded with a silica binder—is taperedfrom the bottom toward the opening so that the opening area becomeslarger than the bottom area as shown in the cross section in FIG. 3.Because the stress generated in the silicon ingot is more reduced, alarge grain size silicon ingot with few cracks and dislocations can beproduced.

[0026] The inner space of the mold for producing the siliconingot—having a layered structure comprising the inner silica layer 3containing at least one layer in which the fused silica powder (notshown in the drawing) with a particle size of 100 μm or less and thefine fused silica sand 31 with a particle size of 100-300 μm are bondedwith a binder, and an outer silica layer 4 containing at least one layerin which the fused silica powder with a particle size of 100 μm or lessand the coarse fused silica sand 41 with a particle size of 500-1500 μmis bonded with a silica binder—may have not only a circular crosssection but also a polygonal cross section.

[0027] Since the coarse fused silica sand never makes a direct contactwith the molten silicon, a used fused silica sand or a low purity fusedsilica sand may be used.

[0028] Since the released silica sand has a smaller specific gravitythan the molten silicon, the former is released to outside as SiO gas byreacting with the molten silicon after being separated on the surface ofthe molten silica, leaving no defects within the silicon ingot.

[0029] The mold for producing the silicon ingot, based on the concept ashitherto described, has a layered structure comprising an inner silicalayer containing at least one layer in which a fused silica powder witha particle size of 100 μm or less and a fine fused silica sand with aparticle size of 100-300 μm is bonded with a silica binder, and an outersilica layer containing at least one layer in which a fused silicapowder with a particle size of 100 μm or less and a coarse fused silicasand with a particle size of 500-1500 μm is bonded with a silica binder.

[0030] The wall face of the inner space of the mold for producing asilicon ingot is tapered from the bottom to the opening so that theopening area becomes larger than the bottom area.

[0031] The mold for producing a silicon ingot has an inner space whosehorizontal cross section assumes a circle or a polygon.

[0032] The wall face of the inner space of the mold for producing asilicon ingot is tapered from the bottom toward the opening so that theopening area becomes larger than the bottom area and the mold has aninner space whose horizontal cross section assumes a circle or apolygon.

[0033] The method for fabricating a mold for producing a silicon ingot,based on the concept as hitherto described, having an inner silica layerand an outer silica layer comprises the steps of:

[0034] forming a slurry layer on the surface of a wax mold by immersing,followed by lifting up, the wax mold having the same shape as that ofthe inner space of the mold for producing a silicon ingot into a slurrycomprising fused silica powder and colloidal silica, followed by formingan inner stucco layer by sprinkling a fine fused silica sand with aparticle size of 100-300 μm on the surface of the slurry layer;

[0035] forming an outer stucco layer on the inner stucco layer bysprinkling the coarse fused silica sand with a particle size of 500-1500μm on the surface of the slurry layer after forming the slurry layer byfurther dipping, followed by lifting up, the wax mold on which the innerstucco layer has been formed into a slurry comprising the fused silicapowder and colloidal silica; and

[0036] heat-melting and eliminating the wax mold along with by bakingthe inner stucco layer and outer stucco layer.

[0037] The method for fabricating a mold for producing a silicon ingothaving an inner silica layer and an outer silica layer comprises thesteps of:

[0038] forming a slurry layer on the surface of a wax mold by dipping,followed by lifting up, a wax mold having the same shape as that of theinner shape of the mold for producing a silicon ingot into a slurrycomprising a fused silica powder and a colloidal silica, at least onelayer of an inner stucco layer being formed by applying at least oneprocess for forming the inner stucco layer by sprinkling a fine fusedsilica sand with a particle size of 100-300 μm on the surface of theslurry layer;

[0039] forming at least one outer stucco layer on the inner stucco layerby applying at least one process for forming an outer stucco layer bysprinkling the coarse fused silica sand with a particle size of 500-1500μm on the surface of the slurry layer after forming the slurry layer byfurther dipping, followed by lifting up, the wax mold on which the innerstucco layer has been formed into a slurry comprising the fused silicapowder and colloidal silica; and

[0040] heat-melting and eliminating the wax mold along with baking theinner stucco layer and outer stucco layer.

[0041] In the method for fabricating a mold for producing a siliconingot as hitherto described, the wall face of the wax mold is taperedfrom the bottom toward the top so that the top area becomes larger thanthe bottom area.

[0042] In the method for fabricating a mold for producing a siliconingot, the wall face of the wax mold is tapered from the bottom towardthe top so that the top area becomes larger than the bottom area and thewax mold has a horizontal cross section assuming a circle or a polygon.

[0043] The particle size of the fused silica sand contained in the innerlayer of the mold for producing a silicon ingot according to the presentinvention was limited to 100 μm or less because the fused silica powdermixed with the colloidal silica is precipitated and it becomes difficultto uniformly disperse the fused silica powder when the fused silicapowder has a coarse particle size of 100 μm or more.

[0044] The particle size of the fine fused silica sand contained in theinner silica layer is limited in the range of 100-300 μm because theinner silica layer becomes difficult to release from the outer silicalayer when the fine silica sand has a particle size as coarse as morethan 300 μm while, when the fine fused silica sand contained in theinner layer has a particle size as fine as less than 50 μm, the innersilica layer becomes so readily peeled that it is released whileproducing the mold. In addition, the particle size of the coarse fusedsilica sand contained in the outer silica layer of the mold forproducing a silicon ingot according to the present invention is limitedin the range of 500-1500 μm because, when the coarse fused silica sandhas a particle size as coarse as more than 1500 μm, the specific gravityof the mold is decreased thereby lowering its strength while, when thecoarse fused silica sand has a particle size as fine as less than 500μm, the strength of the outer silica layer is lowered along withdeteriorating detachability of the outer silica layer from the innersilica layer.

[0045] As shown in FIG. 3, the mold for producing the silicon ingotaccording to the present invention has a layered structure comprising aninner silica layer containing at least one layer in which a fused silicapowder with a particle size of 100 μm or less and a fine fused silicasand with a particle size of 100-300 μm is bonded with a silica binder,and an outer silica layer containing at least one layer in which a fusedsilica powder with a particle size of 100 μm or less and a coarse fusedsilica sand with a particle size of 500-1500 μm is bonded with a silicabinder. It is preferable to provide a taper angle θ at the wall face ofthe inner shape of the mold for producing the silicon ingot from thebottom toward the opening so that the opening area becomes larger thanthe bottom area since the grain size can be developed coarse duringsolidification of the molten silicon owing to an expanded cross sectionat the solidification boundary. The taper angle is preferably in therange of 1-5° C.

[0046] It is preferable that the inner silica layer, containing at leastone layer in which a fused silica powder with a particle size of 100 μmor less and a fine fused silica sand with a particle size of 100-300 μmis bonded with a silica binder, has a thickness that allows the innersilica layer to be released by shrinking of the silicon ingot duringsolidification. The preferred thickness is in the range of 0.1-5 mm.Also, the outer silica layer, containing at least one layer in which thefused silica powder with a particle size of 100 μm or less and thecoarse fused silica sand with a particle size of 500-1500 μm are bondedwith a binder, is required to have a thickness of about 5 mm formaintaining the strength of the mold in producing the silicon ingot.

[0047] However, since too thick layer results in high production cost,the practical thickness is preferably in the range of 3-20 mm.

[0048] The inner silica layer constructing the mold for producing theingot according to the present invention is formed by forming a slurrylayer on the surface of a wax mold by dipping, followed by lifting up, awax mold having the same shape as that of the inner shape of the moldfor producing a silicon ingot into a slurry comprising a fused silicapowder and colloidal silica, at least one layer of the inner stuccolayer being formed by applying at least one process for forming theinner stucco layer by sprinkling the fine fused silica sand with aparticle size of 100-300 μm on the surface of the slurry layer. Theouter silica layer constructing the mold for producing the ingotaccording to the present invention is formed by forming at least oneouter stucco layer on the inner stucco layer by applying at least oneprocess for forming an outer stucco layer by sprinkling the coarse fusedsilica sand with a particle size of 500-1500 μm on the surface of theslurry layer after forming the slurry layer by further dipping, followedby lifting up, the wax mold on which the inner stucco layer has beenformed with a slurry comprising the fused silica powder and colloidalsilica.

[0049] The inner shape of the mold for producing the silicon ingotaccording to the present invention can be not only fabricated to be aconventional circle, but also to be polygonal in its horizontalcross-section at an arbitrary height of the mold for producing thesilicon ingot as shown in FIG. 4.

[0050] The mold for producing the silicon ingot according to the presentinvention having a space with an arbitrary size and shape of the mold(for example, a circular column, a hexagonal column, a cubic or arectangular column) is fabricated by the steps comprising: fabricating awax mold having an arbitrary size and shape (for example, a circularcolumn, a hexagonal column, a cube and a rectangular column); forming aslurry layer on the surface of the wax mold by dipping, followed bylifting up, the wax mold into a slurry comprising a fused silica powderwith a particle size of 100 μm or less and a colloidal silica;sprinkling a fine fused silica sand with a particle size of 100-300 μmon the surface of the slurry layer; forming an inner silica layer byapplying at least one process for forming an outer stucco layer; forminga surface slurry layer of the inner silica layer by dipping, followed bylifting up, the wax mold (having a shape of a circular column, hexagonalcolumn, cube or rectangular column) after forming the inner silica layerinto a slurry comprising fused silica powder with a diameter of 100 μmand colloidal silica;

[0051] sprinkling coarse fused silica sand with a particle size of500-1500 μm on the surface of the slurry layer; and forming the outersilica layer by applying at least one process for forming the stuccolayer.

[0052] The inner shape of the silicon ingot having a square orrectangular cross section can be obtained by using the mold forproducing the silicon ingot with a cubic or rectangular column innershape according to the present invention. Using the silicon ingot havinga square or rectangular cross section especially for producing a squareor rectangular silicon substrate of the photovoltaic solar cell allowsthe expensive silicon ingot to be effectively utilized. When the squareor rectangular silicon substrate for use in the photovoltaic solar cellis produced from the conventional rod-shaped silicon ingot, a silicondisk is firstly produced from the rod-shaped silicon ingot followed bycutting four corners off the silicon disk to form into a square orrectangular silicon substrate for the photovoltaic solar cell, wastingthe cut-off portions from the disk. However, when the silicon ingot hasa square or rectangular cross section, the silicon plate obtained byslicing the silicon ingot also has a square or rectangular shape,thereby extremely saving the cut-off proportion for producing the squareor rectangular silicon substrate for the photovoltaic solar cellenabling to save production costs.

[0053] According to the results obtained by the inventors of the presentinvention, a cheap graphite mold or quartz mold may be used as anoutermost core of the mold. A layer comprising an inner silica layercontaining at least one layer in which the fused silica powder with aparticle size of 100 μm or less and the fine fused silica sand with aparticle size of 100-300 μm are bonded with a binder, or a layercomprising the foregoing inner silica layer and an outer silica layercontaining at least one layer in which the fused silica powder with aparticle size of 100 μm or less and the coarse fused silica sand with aparticle size of 500-1500 μm are bonded with a binder, may be formed onthe inner face of the graphite or quartz mold described above. Since thegraphite mold or the quartz mold serves as a reinforcing outermost corewhen it is used as an outermost core, the foregoing inner silica layeror a layer comprising the inner silica layer and outer silica layer maybe used as an inner layer on the inner face of the graphite mold orquartz mold.

[0054] As shown in the cross section in FIG. 5, the silicon ingot,produced by using a graphite ingot 11 whose inner side is coated with aninner silica layer 3 containing the fused silica powder (not shown inthe drawing) with a particle size of 100 μm or less and the fine fusedsilica sand 31 with a particle size of 100-300 μm, has a puritycomparable to the purity of the silicon ingot produced by using thequartz mold, making it possible to obtain a silicon ingot using thegraphite ingot having a photovoltaic conversion efficiency as excellentas that of the silicon ingot produced by using a quartz mold.

[0055] The inner silica layer 3 containing the fine fused silica sand 31is readily released from the inner wall of the graphite mold 11 in themold for producing the silicon ingot having the inner silica layer 3 asshown by the cross section in FIG. 5. Accordingly, a peeled portion Bappears as shown in the cross section in FIG. 6 by allowing theperiphery of the silicon ingot 2 to be pulled by the inner wall of themold when the solidified silicon ingot shrinks by cooling after themolten silicon is poured into the mold for producing the silicon ingot,thereby leaving no inner stress in the silicon ingot 2. Accordingly, theingot produced as described above never suffers from cracks due tostress as seen in the ingot produced by using a conventional quartzingot.

[0056] The silicon ingot, produced by using the graphite mold in which alayer having an outer silica layer 4 containing the fused silica powder(not shown in the drawing) having a particle size of 100 μm or less andthe inner silica layer 3 containing the fine fused silica sand having aparticle size of 100-300 jim are formed on the inner side of thegraphite mold 11 as shown in FIG. 7, has a purity comparable to thepurity of the ingot produced by using the quartz mold, enabling toproduce a silicon substrate having an excellent photovoltaic conversionefficiency from the silicon ingot.

[0057] The outer silica layer 4 containing the coarse fused silica sand41 is readily released from the inner wall of the graphite mold forproducing the silicon ingot having the inner silica layer 3 and outersilica layer 4 as described above. Accordingly, a peeled portion Bappears as shown in the cross section in FIG. 8 by allowing theperiphery of the silicon ingot 2 to be pulled by the inner wall of themold when the solidified silicon ingot shrinks by cooling after themolten silicon is poured into the mold for producing the silicon ingot,thereby leaving no inner stress in the silicon ingot 2.

[0058] Both the thicknesses of the inner silica layer and outer silicalayer described above may be formed in the range as thin as 1.0-5.0 mmsince the graphite mold serves as a reinforce core. A quartz mold may beused instead of the graphite mold.

[0059] Since the separated silica sand has smaller specific gravity thanthe molten silicon, the silica sand is discharged to outside as SiO gasby reacting with the molten silicon after being separated into thesurface of the molten liquid, leaving no defects in the ingot behind.

[0060] The present invention based on the findings as described aboveprovides a mold for producing a silicon ingot in which an inner silicalayer containing fine fused silica sand with a particle size of 100-300μm is formed on the inner surface of a graphite mold or quartz mold.

[0061] The present invention also provides a mold for producing asilicon ingot in which an inner silica layer containing at least onelayer prepared by binding fused silica powder with a particle size of100 μm or less with fine fused silica sand with a particle size of100-300 Am is formed on the inner surface of a graphite mold or quartzmold.

[0062] The present invention also provides a mold for producing asilicon ingot in which an outer silica layer containing coarse fusedsilica sand with a particle size of 500-1500 μm is formed on the innerface of a graphite mold or a quartz mold, an inner silica layercontaining fine fused silica sand with a particle size of 100-300 μmbeing formed on the inner side of the outer silica layer.

[0063] The present invention also provides a mold for producing asilicon ingot in which an outer silica layer containing at least onelayer prepared by joining fused silica powder with a particle size of100 μm or less and coarse fused silica sand with a particle size of500-1500 μm with a silica binder is formed on the inner face of agraphite mold or quartz mold, an inner silica layer containing at leastone layer prepared by joining fused silica powder with a particle sizeof 100 μm or less and fine fused silica sand with a particle size of100-300 μm with a silica binder being formed on the outer silica layer.

[0064] The present invention al so provides a mold for producing asilicon ingot in which the wall face of the mold is tapered from thebottom toward the opening so that the opening area becomes larger thanthe bottom area.

[0065] The present invention also provides a mold for producing asilicon ingot in which the horizontal cross section of the mold has aninner space assuming a circle or a polygon.

[0066] The present invention also provides a mold for producing asilicon ingot in which the wall face of the mold is tapered from thebottom toward the opening so that the opening area becomes larger thanthe bottom area and the horizontal cross section of the mold has aninner space assuming a circle or a polygon.

[0067] The present invention provides a method for fabricating a moldfor producing a silicon ingot, wherein a stucco layer is formed bysprinkling, followed by baking, fine fused silica sand with a particlesize of 100-300 μm on the surface of a slurry layer after forming theslurry layer on the inner face of the mold by coating or spraying theslurry comprising fused silica powder with a particle size of 100 μm orless and colloidal silica on inside of a graphite mold or a quartz mold.

[0068] The present invention also provides a method for fabricating amold for producing a silicon ingot comprising the steps of:

[0069] forming an outer stucco layer by sprinkling coarse fused silicasand with a particle size of 500-1500 μm on the surface of a slurrylayer after forming the slurry layer on the inner face of the mold bycoating or spraying a slurry comprising fused silica powder with aparticle size of 100 μm or less and colloidal silica on inside of agraphite mold or a quartz mold; and

[0070] forming a stucco layer by sprinkling, followed by baking, finefused silica sand with a particle size of 100-300 μm on the surface ofthe slurry layer after forming the slurry layer by further coating orspraying the slurry comprising fused silica powder with a particle sizeof 100 μm or less and colloidal silica on the outer stucco layer.

[0071] The present invention also provides a method for fabricating amold for producing a silicon ingot in which the wall face of thegraphite mold or the quartz mold is tapered from the bottom toward theopening so that the opening area becomes larger than the bottom area.

[0072] The present invention also provides a method for fabricating amold for producing a silicon ingot in which the horizontal cross-sectionof the graphite mold or the quartz assumes a circle or a polygon.

[0073] The present invention also provides a method for fabricating amold for producing a silicon ingot in which the inner wall face of thegraphite mold or the quartz mold is tapered from the bottom toward theopening so that the opening area becomes larger than the bottom area andthe horizontal cross-section of the graphite mold or the quartz moldassumes a circle or a polygon.

[0074] The inner silica layer of the mold for producing the siliconingot according to the present invention can be formed by the stepscomprising: forming a slurry layer on the inner surface of the quartzmold or graphite mold by coating or spraying a slurry comprising a fusedsilica powder with a mean particle size of 100 μm or less and colloidalsilica; and applying one or a plurality of the stucco forming process bysprinkling, followed by baking, a fine fused silica sand with aparticles size of 100-300 μm on the inner surface of the slurry layer toform a stucco layer.

[0075] Forming the stucco layer on the slurry coating layer comprisingfused the silica powder and silica slurry by spraying the fine fusedsilica sand allows defect generation that have been appeared by coatingthe conventional release agent to be avoided.

[0076] The particle size of the fused silica sand contained in the innersilica layer is limited to 100 μm or less in the mold for producing thesilica ingot according to the present invention because, when the fusedsilica powder has a particle size as coarse as more than 100 μm, itbecomes difficult to uniformly sprinkle the powder since the fusedsilica powder mixed with the colloidal silica is precipitated. Likewise,the particle size of the fine fused silica sand contained in the innersilica layer is limited within the range of 100-300 μm because, when thefine fused silica sand has a particle size as coarse as more than 300μm, the surface roughness of the inner silica layer becomes too largewhile, when the fine fused silica sand has a particle size as fine asless than 100 μm, the inner silica layer is insufficiently released fromthe mold as well as making it impossible to maintain a sufficientthickness.

[0077] The mold for producing the silicon ingot according to the presentinvention can be also fabricated through the steps comprising: forming aslurry layer by coating or spraying a slurry comprising a fused silicapowder with a particle size of 100 μm or less and a colloidal silica onthe inner face of the graphite mold or quartz mold; applying one or aplurality of stucco layer forming processes by sprinkling a coarse fusedsilica sand with a particle size of 500-1500 μm on the surface of theslurry layer; forming a slurry layer by coating or spraying a slurrycomprising a fused silica powder with a particle size of 100 μm or lessand a colloidal silica on the inner surface of the graphite mold on thestucco layer; applying one or a plurality of stucco layer formingprocesses by sprinkling, followed by baking, a fine fused silica sandwith a particle size of 100-300 μm on the surface of the slurry layer. Athick outer silica layer can be formed by sprinkling the coarse fusedsilica sand, allowing a layer comprising the outer silica layer andinner silica layer to be formed within a short period of time along withallowing the inner silica layer to be thin. Although a thick and strongouter silica layer can be more simply formed as the particle size of thecoarse fused silica sand to be sprinkled on the surface of the slurrylayer is larger, the surface of the outer silica layer becomes too roughwhen the particle size of the coarse fused silica sand contained in theouter silica layer of the mold for producing the silicon ingot is ascoarse as more than 1500 μm, adversely affecting smoothness of the innersilica layer. When the particle size of the coarse fused silica sandcontained in the outer silica layer is as fine as less than 500 μm, onthe other hand, a strong and thick outer silica layer can not be soeasily formed.

EXAMPLES Example 1

[0078] A wax mold with a diameter of 250 mm and a height of 100 mm wasprepared. A binder comprising 30% by volume of super-fine fused silicapowder with a particle size of 100 Å or less and a balance of water(referred to as a colloidal silica hereinafter) was also prepared toproduce a slurry by mixing 200 parts of the colloidal silica with 200parts of the fused silica powder with a particle size of 40 μm. The waxmold above was dipped into the slurry obtained, followed by lifting up,to form a slurry layer on the surface of the wax mold. Fine fused silicasand with a particle size of 150 μm was sprinkled on the surface of theslurry layer. This process was repeated three times to form a stuccolayer, thereby forming an inner stucco layer comprising three layerswith a combined thickness of 3 mm on the surface of the wax mold.

[0079] The wax mold on the surface of which the inner stucco layer wasformed was dipped into the foregoing slurry again, followed by liftingup, to form a slurry layer on the surface of the inner stucco layer ofthe wax mold. Coarse fused silica sand with a particle size of 1000 umwas sprayed on the surface of the slurry layer and, by forming stuccolayers by repeating the above process eight times, an outer stucco layercomprising eight layers with a combined thickness of 8 mm was formed onthe surface of the wax mold.

[0080] Then, the wax mold whose inner surface was coated with the innerstucco layer and outer stucco layer was removed by allowing the wax tomelt by heating at 100° C. The layered body obtained comprising theinner stucco layer and outer stucco layer was baked by keeping at atemperature of 800° C. for 2 hours, producing a mold for producing thesilicon ingot according to the present invention comprising the innersilica layer and outer silica layer (referred to as a mold according tothe present invention hereinafter).

[0081] A molten silicon held at a temperature of 1500° C. was pouredinto the mold 1 according to the present invention and was cooled at acooling rate of 1° K/sec., producing a columnar silicon ingot byremoving the ingot from the mold 1 according to the present invention.Presence of cracks due to stress and the amount of the remaining releaseagent on the surface of the columnar silicon ingot obtained werevisually observed. Also, the polycrystalline silicon ingot obtained wassliced to produce a silicon substrate with a width of 150 mm and alength of 150 mm for use in the photovoltaic solar cell and thephotovoltaic conversion efficiency of the silicon substrate wasmeasured. The results of the observation and measurement are summarizedin Table 1.

Example 2

[0082] A rectangular, column shaped wax mold with a length of 170 mm, awidth of 170 mm and a height of 100 mm was prepared. The mold 2according to the present invention was fabricated by the method as inExample 1 to produce a rectangular column shaped silicon ingot by thesame method as in Example 1. Presence of cracks due to stress and theamount of the remaining release agent on the surface of the rectangularcolumn shaped silicon ingot obtained were visually observed. Also, thepolycrystalline silicon ingot obtained was sliced to produce a siliconsubstrate with a width of 150 mm and a length of 150 mm for use in thephotovoltaic solar cell and the photovoltaic conversion efficiency ofthe silicon substrate was measured. The results of the observation andmeasurement are summarized in Table 1.

Example 3

[0083] A conical table shaped wax mold with a bottom diameter of 190 mm,a top diameter of 200 mm and a height of 100 mm whose side face istapered at an angle of 3° was prepared. The mold 3 according to thepresent invention was fabricated using the conical table shaped wax moldby the method as in Example 1 to produce a conical table shaped siliconingot by the same method as in Example 1. Presence of cracks due tostress and the amount of the remaining release agent on the surface ofthe rectangular column shaped silicon ingot obtained were visuallyobserved. Also, the polycrystalline silicon ingot obtained was sliced toproduce a silicon substrate with a width of 150 mm and a length of 150mm for use in the photovoltaic solar cell and the photovoltaicconversion efficiency of the silicon substrate was measured. The resultsof the observation and measurement are summarized in Table 1.

Example 4

[0084] A graphite mold with an inner width of 170 mm and inner length of170 mm, an outer width of 190 mm and outer length of 190 mm, and a depthof 150 mm was prepared. Colloidal silica comprising 30% by volume ofsuper-fine fused silica powder and a balance of water was also prepared.A slurry was formed by mixing 100 parts of this colloidal silica with200 parts of fused silica powder having a mean particle size of 40 μm. Aslurry layer was formed by coating the inner face of the graphite moldwith the slurry obtained, followed by forming a stucco layer bysprinkling the fine fused silica sand with a particle size of 150 μm onthe surface of the slurry layer. This process was repeated three timesand an inner silica layer with a combined thickness of 2 mm was formedon the inner face of the graphite mold by baking at a temperature of800° C. for two hours, thereby fabricating the mold 4 according to thepresent invention.

[0085] A molten silicon kept at a temperature of 1500° C. was pouredinto the mold 4 according to the present invention to produce apolycrystalline silicon ingot. Presence of cracks due to stress and theamount of the remaining release agent on the surface of thepolycrystalline silicon ingot obtained were visually observed. Also, thepolycrystalline silicon ingot obtained was sliced to produce a siliconsubstrate with a width of 150 mm and a length of 150 mm for use in thephotovoltaic solar cell and the photovoltaic conversion efficiency ofthe silicon substrate was measured. The results of the observation andmeasurement are summarized in Table 1.

Example 5

[0086] A slurry layer was formed using the graphite mold and slurryprepared in Example 4 by coating the inner face of the graphite moldwith the slurry. The coarse fused silica sand with a particle size of1000 μm was sprinkled on the surface of the slurry layer to form astucco layer by repeating this sprinkling process twice. A slurry layerwas additionally formed by coating the surface of the stucco layer withthe slurry prepared in Example 4. The fine fused silica sand with aparticle size of 150 μm was sprinkled on the slurry layer to form astucco layer by repeating this sprinkling process twice, thereby forminga stucco layer containing coarse fused silica sand and fine fused silicasand. The graphite mold in which the stucco layer was formed was bakedat a temperature of 800° C. for 8 hours in an inert gas atmosphere,fabricating the mold 5 according to the present invention by forming alayer comprising the inner silica layer and outer silica layer with acombined thickness of 3 mm on the inner face of the graphite mold.

[0087] A molten silicon kept at a temperature of 1500° C. was pouredinto the mold 5 according to the present invention and the mold wascooled at a cooling rate of 0.6° C./sec. to produce a polycrystallinesilicon ingot. Presence of cracks due to stress and the amount of theremaining release agent on the surface of the polycrystalline siliconingot obtained were visually observed. Also, the polycrystalline siliconingot obtained was sliced to produce a silicon substrate with a width of150 mm and a length of 150 mm for use in the photovoltaic solar cell andthe photovoltaic conversion efficiency of the silicon substrate wasmeasured. The results of the observation and measurement are summarizedin Table 1.

Conventional Example 1

[0088] For the comparative purpose, a mold prepared by coating Si₃N₄ onthe inner face of the graphite mold as prepared in Example 4 was used asa conventional graphite mold. A molten silicon kept at a temperature of1500° C., the molten silicon prepared in Example 4, was poured into theconventional graphite mold and the mold was cooled at a cooling rate of0.6° C./sec., producing a polycrystalline silicon ingot by removing thesilicon ingot from the conventional graphite mold. Presence of cracksdue to stress and the amount of the remaining release agent on thesurface of the polycrystalline silicon ingot obtained were visuallyobserved. Also, the polycrystalline silicon ingot obtained was sliced toproduce a silicon substrate with a width of 150 mm and a length of 150mm for use in the photovoltaic solar cell and the photovoltaicconversion efficiency of the silicon substrate was measured. The resultsof the observation and measurement are summarized in Table 1.

Conventional Example 2

[0089] For the comparative purpose, a conventional quartz mold with aninner diameter of 240 mm, an outer diameter of 250 mm and a depth of 150mm was prepared. A molten silicon kept at a temperature of 1500° C., themolten silicon prepared in Example 4, was poured into the mold and themold was cooled at a cooling rate of 0.6° C./sec., producing apolycrystalline silicon ingot by removing the silicon ingot from theconventional type quartz mold. Presence of cracks due to stress and theamount of the remaining release agent on the surface of thepolycrystalline silicon ingot obtained were visually observed. Also, thepolycrystalline silicon ingot obtained was sliced to produce a siliconsubstrate with a width of 150 mm and a length of 150 mm for use in thephotovoltaic solar cell and the photovoltaic conversion efficiency ofthe silicon substrate was measured. The results of the observation andmeasurement are summarized in Table 1. TABLE 1 Photovoltaic Residualconversion Kind of mold Stress Cracks release agent efficiency (%) Mold1 of the present None None 14.2 invention Mold 2 of the present NoneNone 13.9 invention Mold 3 of the present None None 14.1 invention Mold4 of the present None None 13.9 invention Mold 5 of the present NoneNone 14.0 invention Conventional graphite None Yes 12.8 moldConventional quartz Yes None 14.2 mold

[0090] It is evident from the results summarized in Table 1 that thepolycrystalline silicon ingot produced by using the molds 1-5 accordingto the present invention has a better photovoltaic conversion efficiencythen the polycrystalline silicon ingot using the conventional graphitemold. Although the polycrystalline silicon ingot produced by using themolds 1-5 according to the present invention has a photovoltaicconversion efficiency comparable to the polycrystalline silicon ingotproduced by using the conventional quartz mold, using the mold 1-5according to the present invention allows production yield of thepolycrystalline silicon ingot to be high, being free from the residualrelease agent.

[0091] As hitherto described, the present invention provides a mold forproducing a silicon ingot with which a polycrystalline silicon ingothaving a good photovoltaic conversion efficiency, being free from theresidual release agent and generating no cracks due to stress can beproduced. In addition, since a silicon ingot having not only a circularcross section but also a polygonal cross section is produced, a squareor rectangular silicon substrate such as a silicon substrate for use inthe photovoltaic solar cell can be produced with no waste of the siliconingot, being effective for mass production and cost down of the siliconsubstrate for use in the photovoltaic solar cell.

[0092] The disclosure of Japanese priority application No. 10-045460filed Feb. 26, 1998 and Japanese priority application No. 10-047449filed Feb. 27, 1998 are hereby incorporated by reference into thepresent application.

[0093] Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is,therefore, to be understood that within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed herein.

What is claimed as new and is intended to be secured by letters patentis:
 1. A mold for producing a silicon ingot having a layered structurecomprising an inner silica layer containing at least one layer in whicha fused silica powder with a particle size of 100 μm or less and a finefused silica sand with a particle size of 100-300 μm is bonded with asilica binder, and an outer silica layer containing at least one layerin which a fused silica powder with a particle size of 100 μm or lessand a coarse fused silica sand with a particle size of 500-1500 μm isbonded with a silica binder.
 2. A mold for producing a silicon ingotaccording to claim 1, wherein the wall face of the inner shape of themold for producing a silicon ingot is tapered from the bottom toward theopening so that the opening area becomes larger than the bottom area. 3.A mold for producing a silicon ingot according to claim 1 having aninner space whose horizontal cross section assumes a circle or apolygon.
 4. A mold for producing a silicon ingot according to claim 1,wherein the wall face of the inner shape of the mold for producing asilicon ingot is tapered from the bottom toward the opening so that theopening area becomes larger than the bottom area and the mold has aninner space whose horizontal cross-section assumes a circle or apolygon.
 5. A method for fabricating a mold for producing a siliconingot having an inner silica layer and an outer silica layer comprisingthe steps of: forming a slurry layer on the surface of a wax mold byimmersing, followed by lifting up, the wax mold having the same shape asthat of the inner shape of the mold for producing a silicon ingot into aslurry comprising a fused silica powder and colloidal silica, followedby forming an inner stucco layer by sprinkling a fine fused silica sandwith a particle size of 100-300 μm on the surface of the slurry layer;forming an outer stucco layer on the inner stucco layer by sprinklingcoarse fused silica sand with a particle size of 500-1500 μm on thesurface of the slurry layer after forming the slurry layer by furtherdipping, followed by lifting up, the wax mold on which the inner stuccolayer has been formed into a slurry comprising the fused silica powderand colloidal silica; and heat-melting and eliminating the wax moldfollowed by baking the inner stucco layer and outer stucco layer.
 6. Amethod for fabricating a mold for producing a silicon ingot having aninner silica layer and an outer silica layer comprising the steps of:forming a slurry layer on the surface of a wax mold by dipping, followedby lifting up, a wax mold having the same shape as that of the innershape of the mold for producing a silicon ingot into a slurry comprisinga fused silica powder and colloidal silica, at least one layer of aninner stucco layer being formed by applying at least one process forforming the inner stucco layer by sprinkling a fine fused silica sandwith a particle size of 100-300 μm on the surface of the slurry layer;forming at least one outer stucco layer on the inner stucco layer byapplying at least one process for forming an outer stucco layer bysprinkling the coarse fused silica sand with a particle size of 500-1500μm on the surface of the slurry layer after forming the slurry layer byfurther dipping, followed by lifting up, the wax mold on which the innerstucco layer has been formed into a slurry comprising the fused silicapowder and colloidal silica; and heat-melting and eliminating the waxmold followed by baking the inner stucco layer and outer stucco layer.7. A method for fabricating a mold for producing a silicon ingotaccording to claim 5 or 6, wherein the wall face of the wax mold istapered from the bottom toward the top so that the top area becomeslarger than the bottom area.
 8. A method for fabricating a mold forproducing a silicon ingot according to claim 5 or 6, wherein the waxmold has a horizontal cross section assuming a circle or a polygon.
 9. Amethod for fabricating a mold for producing a silicon ingot according toclaim 5 or 6, wherein the wall face of the wax mold is tapered from thebottom toward the top so that the top area becomes larger than thebottom area and the wax mold has a horizontal cross-section assuming acircle or a polygon.
 10. A mold for producing a silicon ingot in whichan inner silica layer containing a fine fused silica sand with aparticle size of 100-300 μm is formed on the inner surface of a graphitemold or quartz mold.
 11. A mold for producing a silicon ingot in whichan inner silica layer containing at least one layer prepared by bindinga fused silica powder with a particle size of 100 μm or less and finefused silica sand with a particle size of 100-300 μm with a silicabinder is formed on the inner surface of a graphite mold or quartz mold.12. A mold for producing a silicon ingot in which an outer silica layercontaining a coarse fused silica sand with a particle size of 500-1500μm is formed on the inner face of a graphite mold or a quartz mold, aninner silica layer containing a fine fused silica sand with a particlesize of 100-300 μm being formed on the inner side of the outer silicalayer.
 13. A mold for producing a silicon ingot in which an outer silicalayer containing at least one layer prepared by joining fused silicapowder with a particle size of 100 μm or less and coarse fused silicasand with a particle size of 500-1500 μm with a silica binder is formedon the inner face of a graphite mold or quartz mold, an inner silicalayer containing at least one layer prepared by joining the fused silicapowder with a particle size of 100 μm or less and fine fused silica sandwith a particle size of 100-300 μm with a silica binder being formed onthe outer silica layer.
 14. A mold for producing a silicon ingotaccording to claim 10, 11, 12 or 13, wherein the wall face of the moldfor producing a silicon ingot is tapered from the bottom toward theopening so that the opening area becomes larger than the bottom area.15. A mold for producing a silicon ingot according to claim 10, 11, 12or 13, wherein the horizontal cross section of the mold for producing asilicon ingot has an inner space assuming a circle or a polygon.
 16. Amold for producing a silicon ingot according to claim 10, 11, 12 or 13,wherein the wall face of the mold for producing a silicon ingot istapered from the bottom toward the opening so that the opening areabecomes larger than the bottom area and the horizontal cross section ofthe mold has an inner space assuming a circle or a polygon.
 17. A methodfor fabricating a mold for producing a silicon ingot, wherein a stuccolayer is formed by sprinkling, followed by baking, a fine fused silicasand with a particle size of 100-300 μm on the surface of a slurry layerafter forming the slurry layer on the inner face of the graphite mold orthe quartz mold by coating or spraying the slurry comprising fusedsilica powder with a particle size of 100 μm or less and colloidalsilica.
 18. A method for fabricating a mold for producing a siliconingot comprising the steps of: forming an outer stucco layer bysprinkling a coarse fused silica sand with a particle size of 500-1500μm on the surface of a slurry layer after forming the slurry layer onthe inner face of the mold by coating or spraying a slurry comprisingthe fused silica powder with a particle size of 100 μm or less andcolloidal silica on the inner side of a graphite mold or a quartz mold;and forming a stucco layer by sprinkling, followed by baking, a finefused silica sand with a particle size of 100-300 μm on the surface ofthe slurry layer after forming the slurry layer by further coating orspraying the slurry comprising fused the silica powder with a particlesize of 100 μm or less and colloidal silica on the outer stucco layer.19. A method for fabricating a mold for producing a silicon ingotaccording to claim 17 or 18, wherein the inner wall face of the graphitemold or the quartz mold is tapered from the bottom toward the opening sothat the opening area becomes larger than the bottom area.
 20. A methodfor fabricating a mold for producing a silicon ingot according to claim17 or 18, wherein the horizontal cross-section of the graphite mold orthe quartz mold assumes a circle or a polygon.
 21. A method forfabricating a mold for producing a silicon ingot according to claim 17or 18, wherein the inner wall face of the graphite mold or the quartzmold is tapered from the bottom toward the opening so that the openingarea becomes larger than the bottom area and the horizontalcross-section of the graphite mold or the quartz mold assumes a circleor a polygon.